1. Field of the Invention
The present invention relates to a light signal receiver for converting a modulated light signal to an electrical signal.
2. Related Background Art
A light signal receiver has been widely used as an apparatus for detecting a change in a moving article or a material which exists on an optical path, and recently, it has been used as a light communication apparatus for transmitting various information by optical means by using a space or optical fiber as a transmission medium. A photo-sensitive element such as a photodiode or avalanche photo-diode has been used as a photoelectric conversion element for converting the light signal to the electrical signal.
FIG. 3 shows a specific circuit of a prior art light signal receiver.
In FIG. 3, numeral 1 denotes a photo-diode. A backward voltage +B is applied to a cathode of the photo-diode 1, and a load resistor 3 of the photo-diode 1 is connected to an anode. The junction between the photodiode 1 and the load resistor 3 is connected to an input terminal of an amplifier 2 through a coupling capacitor 5. Numeral 6 denotes an output terminal of the amplifier 2 and numeral 4 denotes a bypass capacitor for the backward voltage +B. In this light signal receiver, a light signal applied to the photo-diode 1 is photo-electrically converted to a signal current which flows through the load resistor 3 to develop a voltage across the load resistor 3. The developed voltage is applied to the amplifier 2 through the coupling capacitor 5, where it is amplified and a signal output is produced at the output terminal 6. The input impedance of the amplifier 2 is sufficiently larger than the load resistor 3.
When the light signal is to be detected in such a circuit, it is necessary to increase the load resistor 3 in order to improve receiving sensitivity. However, it has been experimentarily proved that the sensitivity saturates as the load resistor 3 increases, as shown in FIG. 4.
Further, because of a junction capacitance C.sub.j (shown by broken lines in FIG. 3) of the photo-diode 1, the signal output level decreases as the signal frequency increases, and this leads to reduction of the sensitivity of the light signal receiver.